Undoubtedly you have committed a grievous sin, in fact a number of sins, starting with your speaking to this man about these sexual matters, and ending with what you did to treat him. The Shaytaan has gradually led you astray and made you fall into evil; he has deceived you and made it look to you as if you are helping others and being kind to them.
No matter how soft-hearted a person may be, no matter how sensitive towards others, there are limits that must not be crossed.
We understand from your question that you work in a place where you mix with men. This is where evil begins. Perhaps this is an example of the evil consequences of women mixing with men at work. See also question no. 1200 and 6666.
You should note that it is not permissible for women to speak with men except for necessary purposes, when it should be done in a decorous and dignified manner, without speaking in a soft tone, as Allaah says (interpretation of the meaning):
“then be not soft in speech, lest he in whose heart is a disease (of hypocrisy, or evil desire for adultery) should be moved with desire, but speak in an honourable manner”
If speaking in a soft manner is forbidden, then what about a woman speaking to a man about sex and sexual diseases?!
You must strive to leave this mixed work place and cut off all ties with this man whose lack of religious commitment is worse than any physical problems he may be suffering.
With regard to the question of whether what you have done is zina or not, the answer is what the Prophet (peace and blessings of Allaah be upon him) said, according to the hadeeth narrated by al-Bukhaari (5744) and Muslim (4801) from Abu Hurayrah: “Allaah has decreed for the son of Adam his share of zina, which will inevitably catch up with him. The zina of the eyes is looking and the zina of the tongue is speaking. The nafs (self) wishes and desires, and the private parts confirm that or deny it.”
The meaning of this hadeeth is that the son of Adam has his share of zina decreed for him. Some of them commit zina in real terms, by engaging in illicit sexual intercourse. Some of them commit zina in a figurative sense by looking at haraam things or listening to stories or songs of zina and anything that may lead to it, or by touching or kissing a non-mahram woman, or by speaking in a haraam fashion with a non-mahram woman, and so on. These are all kinds of figurative zina, and the private part confirms that or denies it meaning that zina with the private parts may take place or may not. And Allaah knows best.
But by Allaah’s mercy towards His slaves He opens to them the door of repentance and seeking forgiveness, as He says (interpretation of the meaning):
“And verily, I am indeed forgiving to him
who repents, believes (in My Oneness, and associates none in worship with Me) and does righteous good deeds, and then remains constant in doing
them (till his death)”
“And whosoever repents and does righteous good deeds; then verily, he repents towards Allaah with true repentance”
You said that you have repented to Allaah, so do a lot of righteous deeds, so that Allaah may forgive you. Part of repentance means cutting all ties with this man and asking to be transferred away from the place where he works, indeed to be transferred away from a mixed workplace altogether, as stated above.
And Allaah knows best.